Rules for detecting "out-of-control" or non-random conditions were first postulated by Walter A. Shewhart[1] in the 1920s.
[2] The rules are applied to a control chart on which the magnitude of some variable is plotted against time.
The rules are based on the mean value and the standard deviation of the samples.
Applying these rules indicates when a potential "out of control" situation has arisen.
Equally there may be some missing alerts where some specific "out of control" situation is not detected.